WASHINGTON  Every presidential candidate makes promises. What's rare about this week's turn of events is that they're trying to make policy.

The remarkable joint appearance Thursday by John McCain and Barack Obama at the White House is the result of a confluence of events: the vast financial challenges facing the country, President Bush's political weakness, the ability of both senators to win support from colleagues — and the consequences that the government's response to Wall Street's meltdown will have for the next White House.

So on a day both had planned to be preparing for combat at Friday night's opening debate — Bush noted wryly that their attention had been focused elsewhere — they instead were seated at opposite ends of the table in the Cabinet Room one will inherit. "They were very courteous with each other and very respectful," said Sen. Richard Shelby, R-Ala. — though they haven't yet agreed on the precise prescription.

"It certainly is something you don't ordinarily see," says presidential biographer Robert Dallek, who found no ready parallel since at least World War II. Presidential candidates more often are at odds as they jockey for political advantage — during the Iranian hostage crisis in 1980, for instance, and while the Vietnam peace talks were proceeding in Paris in 1968.

Contributing to the unusual dynamic this year: Neither the president nor vice president is on the ballot, the first time that's happened since 1952. Without the candidates' involvement, the winner would be handed a deal he had little role in crafting.

"There's no question that the next president and the next Treasury secretary will have a lot of say in how this plan is implemented," says Jason Furman, Obama's economic adviser.

McCain and Obama were invited into the talks not to negotiate details but to give any bailout enough political momentum to pass. Both said later in TV interviews that a deal was possible soon, though McCain said Republicans' concerns about the bailout's $700 billion price tag and structure had to be addressed.

"They are not shaping this bill; they are providing political cover for it," says Tom Mann, a political analyst at the Brookings Institution. "McCain's support will help garner some Republican votes that President Bush can't because of his discredited political standing, and similarly Obama is there to lay out broad agreement on principles."

Insiders differ on how critical their physical presence was. Senate Democratic leader Harry Reid, sitting to Bush's left during the meeting, labeled McCain's dramatic return to Washington on Thursday a political stunt. McCain spokesman Tucker Bounds called it critical in averting "a serious cataclysmic-type meltdown."

In any case, their open opposition, delivered in person or on the campaign trail, could have torpedoed the plan.

"If either John McCain or Barack Obama came out and said they opposed it, it would not pass," Ari Fleischer, a veteran Capitol Hill staffer who also served as Bush's White House spokesman, says flatly.

Alice Rivlin, a former budget director in the Clinton administration, says their involvement is "all about confidence."

"It's about confidence of the markets and confidence of the country," she says. "If they weren't so visibly involved, one might say, 'Now that he's president, is he going to do something different?' "

Rivlin notes that President Clinton was able to hit the ground running on economic policy in 1993 because he had convened hours of meetings with experts in Little Rock after the election. Still, Clinton complained that once in office he discovered a bigger deficit than he expected, forcing him to shelve some of his campaign promises.

Because McCain and Obama have been closely involved this time, says House Budget Chairman John Spratt, D-S.C., the next president "doesn't have the argument he didn't see it coming."

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